1. The Field of the Invention
The present invention relates to communication devices. Particularly, the present invention relates to a method and apparatus for determining if a plurality of communication devices exist within a defined geographic area and for identifying the communication devices.
2. The Relevant Art
There are known in the art of communication devices, systems for locating and identifying objects or tags that exist within a defined area or field of coverage. The systems serve varying purposes, ranging from protecting assets in a store to recognizing for safety reasons when persons who are mentally challenged enter a particular locale.
Locating and identifying objects in a field of coverage presents particular difficulties for communication systems where a significant quantity of unknown devices must be located, and where each device requires a significantly lengthy unique identification number. At any given point in such systems, there may be numerous slave communication devices within range of a master communication device, with the slave communication devices moving into and out of the field of coverage either simultaneously or independently.
One method to obtain the identification number of the slaves in a master/slave communication system is to sequentially scroll through each possible identification number and interrogate all possible slaves. However, if the possible combination of addresses is great, this process will be extremely cumbersome. For example, in a system having slaves with 24 bit identifiers, the potential number of addresses is 16 million (224).
Alternatively, a slave entering a field, upon detecting that it is within range of the field, could immediately send its identification number to a master. Since multiple tags might enter the field at one time, however, their messages might collide and never reach the master.
Other approaches in the art describe various methods for locating and identifying objects in a field of coverage and for handling collision when multiple objects in a field respond to a single remote interrogator.
U.S. Pat. No. 5,751,570 issued to Stobbe et al. discloses a method of automatically identifying an unknown number of slave devices in a field. The slave devices transmit their corresponding identities to an interrogator. Particularly, the slave devices are transponders that are activated to send data when they are within the electromagnetic field of a reader. When more than one transponder responds upon being activated, all transponders are put into a dead state by a collision signal transmitted by the reader, thereby ceasing transmission of transponder data. Each transponder thereafter generates a random dead state for itself after receiving the collision signal, with each transponder resuming transmission of its transponder data at the end of the dead state. After receiving the data from a transponder as a result of the deviating dead states, the reader transmits an occupied signal to put the remaining transponders in an idle state in which they no longer transmit transponder data due to the occupied signal. After reception of the complete transponder data from the individual transponder, the reader transmits an acknowledge signal to place the transponder into an idle state. The other transponders are reactivated and the process repeated until all transponders have been identified individually and successively by the reader. Such a system would not be efficient in identifying and locating a significant number of transponders having significantly lengthy identification numbers due to the numerous random period dead states required after collision.
U.S. Pat. 5,686,902 issued to Reis et al. discloses a communication device or method wherein an algorithm is used to reduce collision probability between tags. The system provides bit code sequencing techniques during the identification process. Reis requires shorter identification transmission times, as compared to the overall listening time, to reduce the likelihood of collision between tags. If there are a significant number of tags, however, this method requires extremely long listening periods which creates a problem if tags are moving in and out of the field of coverage. Some tags, for example, will have the opportunity to enter the field and leave before the listening period is over. Also, while Reis shortens the listening period as tags are successfully identified, there is still significant inefficiency when there are few tags to be identified and the listening period remains relatively long.
U.S. Pat. No. 5,030,807 issued to Landt et al. discloses an RF tag identification system wherein an interrogator unit is able to identify a tag that enters a field of coverage. The interrogation device provides a bit by bit identification scheme. Particularly, an interrogator sends an RF signal to a remote object where the signal includes data intended to be received and stored by the remote object. Upon receipt of the RF signal, one or more remote objects return a backscatter-modulated signal to the interrogator, the backscatter-modulated signal being modulated with data indicating the identity and other data stored in the remote object. The interrogator has the capability to recognize the identity and other data stored in the remote object from the returned backscatter-modulated signal. The interrogator also has the capability to transmit data to the identified remote object. Landt uses time division multiplexing and multiple frequency schemes to separate tag-to-interrogator and interrogator-to-tag communications, and to separate communications where multiple, interrogators communicate in close proximity with one another. Time-division multiplexing, however, involves an automatic xe2x80x9cbuilt-inxe2x80x9d delay as each tag in a system must respond, one after another, in time during an interrogation. This built-in delay would be inefficient in a system requiring location and identification. of many unknown tags in a field of coverage at one time. Furthermore, using multiple frequency tags is impractical in a system with numerous tags., Also, while Landt focuses on collision avoidance, Landt does not directly provide a solution when there is- an actual collision between two or more tags responding to the same interrogator.
U.S. Pat. No. 5,673,037 issued to Cesar et al. discloses a method of selecting RF tags for querying, communicating, and/or identifying by an interrogator. The method uses a bit code type control logic algorithm to accomplish these functions. The tags in Cesar are selected and identified in specific groups based on known criteria. Thus, no method for identifying and locating individual unknown tags in a field of coverage is disclosed.
None of the above mentioned prior art systems describe a system or method to effectively handle the situation where numerous unknown slave communication devices enter a field of coverage and thereafter begin providing their respective unique identification numbers simultaneously, thus colliding.
Accordingly, it would be desirable to have a communication system and method for locating and identifying a significant number of unknown devices having unique identification numbers within a field of coverage in an efficient and reliable manner. Particularly, it would be desirable for the same system and method to have an anti-collision feature allowing numerous slave communication devices to simply and efficiently provide unique identification numbers to a master communication device.
The foregoing desired aspects and advantages are achieved in a system of communicating between a master communication device and at least one slave communication device to determine if there is at least one slave device within a defined geographic area and to identify the slave(s). The system comprises a master communication device that establishes a field of coverage and initiates a request to determine if there are any slave communication devices within the field of coverage. Each slave communication device has a unique binary identification number. After receiving the request, slaves in the defined geographic areas send a response to the master indicating that the. slave is within the defined geographic area. The master after receiving, one or more responses from slaves in the defined geographic area, sends a command to the responding slaves to provide their unique binary identification numbers. After receiving the command, the responding slaves begin providing information representative of bits of their respective unique binary identification numbers to the master.
It is another aspect of the present invention for the information representative of bits of the unique binary identification numbers to be provided by responding slaves within first and second time periods of a plurality of time windows.
It is a further aspect of the present invention for the information representative of bits of the unique binary identification numbers to be provided by responding slaves within the first time period of the time windows to designate a bit that has a first binary value, and within the second time period of the time windows to designate a bit having a second binary value, where the second predetermined time period starts after the first predetermined time period has ended.
It is yet a further aspect of the present invention for the master to send an acknowledgment within a predetermined time period after slave responses received during the first and second time periods of the time windows.
Still another aspect of the present invention is where a first slave responds to the master""s command within the first time period of a time window, and a second slave responds to the master""s command within the second time period of the time window, and further where only the first slave receives an acknowledgment from the master, the acknowledgment being sent within a predetermined time period after the response from the first slave during the first time period of the window.
Still yet another aspect of the present invention is where a first slave responds to the master within the first or second time period of a time window, and a second slave responds to the master within the same time period of the window, and where both the first and second slaves receive an acknowledgment from the master, the acknowledgment being sent within a predetermined time period after the response from the first and second slaves.
Still a further aspect of the present invention is where the first slave, after receiving the acknowledgment from the master, provides information representative of bits of its unique binary identification number during subsequent time windows.
Other aspects and advantages of the present invention will become apparent when the description of specific embodiments below are read in conjunction with the accompanying figures.